Electro-pneumatic converters (“I/P converters”) use a signal, such as electrical current, to regulate a fluid pressure. The I/P converters may be mounted to a conduit carrying pressurized fluid. The I/P converters may use the signal to change the size of the opening to allow fluid to escape the pipe. Due to the fluid leaving the conduit, the fluid pressure in the conduit may drop to a desired level. The size of the opening, and therefore the pressure, may be correlated with the signal. For example, the pressure in the conduit is often linearly correlated with a current under ideal conditions. A user may therefore send a signal of, for example, 4-20 milliamps (mA) to the I/P converter and expect that the fluid pressure be at a correlated pressure.
The I/P converters sometimes employ a flapper-nozzle arrangement to control the size of the opening. In such an arrangement the flapper presses against or is proximate to a nozzle with a gap. The size of the gap may be regulated to control the pressure of the fluid in the conduit. When the signal is zero (e.g., zero amps) and there is no fluid pressure, the flapper will be pressed against the nozzle by the weight of the flapper. However, it may be desirable for the flapper to not press against the nozzle when there is no fluid pressure and signal. For example, it may be desirable that zero current correlates with zero fluid pressure. To achieve this zero pressure to zero current correlation, the weight of the flapper is typically countered.
To counter the weight of the flapper, a spring or its equivalent is oftentimes coupled to the flapper. The spring's stiffness can be selected or designed to counter the weight of the flapper so, ideally, zero pressure correlates with the zero current when the flapper is horizontal with respect to gravity. Such an exemplary prior art I/P converter 10 with a spring 12 is shown in FIG. 1. As can be seen, a nozzle 14 is proximate a flapper 16 which is retained by a screw 18. The spring 12 is used to maintain a gap between the nozzle 14 and the flapper 16. That is, without the spring 12, the flapper 16 will press into the nozzle 14 in the absence of pressurized fluid. There may also be friction forces between the screw 18 and the flapper 16 that oppose movement of the flapper 16.
A problem with the I/P converter 10 is that the spring's 12 stiffness can depend on environmental factors such as the temperature, fatigue, corrosion, or the like. Also, the friction forces between the screw 18 and the flapper 16 impedes the movement of the flapper. These friction forces may also change due to the environmental factors. Moreover, other prior art I/P converters typically include bellows, retaining rings, or other parts that are also susceptible to environmental variables and friction forces. The I/P converter 10 is also unable to maintain the correlation between zero fluid pressure and zero current in various orientations. For example, turning the I/P converter 10 upside down causes a significant deviation in the fluid pressure and current correlation. The environmental variables, friction forces, and changes in orientation can therefore cause the correlation between the current and the fluid pressure (signal-pressure correlation) to change or drift over time. As a result, the actual fluid pressure output will differ from an expected or desired fluid pressure.
In addition, the flapper 16 can sometimes be deformed, for example, by the nozzle 14 when the flapper 16 touches the nozzle 14. The flapper might also not lie flat against the nozzle. These imperfections are problematic because the correlation between the gap size and fluid pressure may be a large number. That is, a small increase in the gap size can result in the large fluid pressure drop. Compounding this problem is that the gap between the flapper and nozzle is typically about 30 μm; a very small gap. Therefore, even small deformations or misalignment of the flapper 16 and the nozzle 14 can cause significant deviation in signal-pressure correlation. Moreover, manufacturing a flapper 16 that is within, for example, a few microns in tolerance is prohibitively expensive.
Accordingly, there is a need for I/P converters that do not rely on a material's stiffness to counter the weight of the flapper and have minimal frictional forces. There is also a need for I/P converters with flappers that are aligned and not easily deformed and yet still inexpensive to manufacture.